Polarization dependence of the second harmonic generation (SHG) was studied on a single crystal of the photochromic N-(3,5-di-tertbutylsalicylidene)-4-aminopyridine (4P). The optimal polarization condition for SHG is obtained when both the fundamental and second harmonic waves are parallel to the main axis of the crystal. This experimental result was correlated with calculations based on X-ray diffraction structure. Also, wavelength dependent studies showed that SHG-switching on near-UV irradiation could be tuned by utilizing the absorption and resonance effects of the photo-induced species. (c) 2007 Elsevier B.V. All rights reserved.